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The Role of Clouds in Shifting the Mid-Latitude Jet

Science

A common feature of climate model projections of future changes in cloud-radiative effects is a dipole in the mid-latitudes of the southern hemisphere, with cloud-induced heating on the equatorward side of the jet stream and cloud-induced cooling on the poleward side. This is commonly attributed to the robust poleward shift of the jet stream, which presumably moves clouds from lower latitudes to higher latitudes. However, the direction of causality may be reversed: Cloud-radiative anomalies can alter the surface temperature gradient in mid-latitudes, thereby changing the location of the jet stream. Here we use CMIP5 models to investigate whether future cloud shifts are driven by jet shifts, or vice versa.

Approach

A strong relationship is found between changes in the latitudinal gradient of absorbed shortwave radiation (ASR) and Southern Hemispheric jet shifts in 21st century climate simulations of CMIP5 (Coupled Model Intercomparison Project phase 5) coupled models. The relationship is such that models with increases in the ASR gradient around the southern midlatitudes, and therefore increases in midlatitude baroclinicity, tend to produce a larger poleward jet shift. The ASR changes are shown to be dominated by changes in cloud properties. When we consider experiments in which sea surface temperature increases are prescribed and cloud-radiation anomalies are not allowed to cause changes in surface temperature, we find that the inter-model spread in jet shifts decreases substantially while the spread in ASR gradient anomalies remains large, and the relationship between jet shifts and ASR gradient anomalies disappears. Thus, the ASR changes are the cause, and not the result, of the intermodel differences in jet response.

Impact

Contrary to prevailing wisdom, we demonstrate that ASR gradient changes are the cause, and not the result, of the inter-model differences in jet response. Rather than being driven by jet shifts, future changes in the ASR gradient appear to arise from intrinsic sensitivity of cloud properties to planetary warming that is model-specific. Our results highlight the importance of reducing the uncertainty in cloud feedbacks in order to constrain future circulation changes. This is especially important given the large uncertainty in how much the midlatitude jet will shift poleward in the future.

Acknowledgments

We thank the reviewers (Ed Gerber and anonymous) for their helpful comments. P.C. and D.L.H were supported by the National Science Foundation under grant AGS-0960497. M.D.Z.’s contribution was performed under the auspices of U.S. Department of Energy (DOE) by Lawrence Livermore National Laboratory under contract DE-AC52-07NA27344 and was supported by the Regional and Global Climate Modeling program of the U.S. DOE’s Office of Science. We acknowledge the World Climate Research Programme’s Working Group on Coupled Modelling, which is responsible for CMIP, and we thank the climate modeling groups (listed in Table S1 of this paper) for producing and making available their model output. For CMIP the U.S. Department of Energy’s Program for Climate Model Diagnosis and Intercomparison provides coordinating support and led development of software infrastructure in partnership with the Global Organization for Earth System Science Portals.